[R] Fourier Transform fft help

[stephen sefick]

why not try spectrum (power spectrum)
spectrum(y, log="no")

the major spike is at 0.1 and 1/0.1 = 10Hz

On Wed, Sep 16, 2009 at 12:13 PM, delic <kranius at optonline.net> wrote:
>
> I wrote a script that I anticipating seeing a spike at  10Hz with the
> function 10* sin(2*pi*10*t).
> I can't figure out why my plots  do not show spikes at the frequencies I
> expect. Am I doing something wrong or is my expectations wrong?
>
>
> require(stats)
> layout(matrix(c(1,2,3), 3, 1, byrow = TRUE))
> #SETUP
>   n    <- 256 #nextn(1001) gives next power 2
>   F    <- 100 #Hz -50 to 50 Hz
>   dt   <- 1/F
>   T    <- n*dt
>   df   <- 1/T
>   t    <- seq(0,T,by=dt)  #also try ts function
>   t<-t[1:length(t)-1]
>   freq <- 5 #Hz
>
> #SIGNAL FUNCTION
>   y     <- 10* sin(2*pi*10*t)   #10*sin(2*pi*freq*t)
> #FREQ ARRAY
>   #f     <- seq(-F/2,F/2,by=df)
>        f     <- F/2*seq(-1,1,length.out=n+1)
>   f<-f[1:length(f)-1]
> #FOURIER WORK
>   Y     <- fft(y)
>   mag   <- sqrt(Re(Y)^2+Im(Y)^2)
>   phase <- atan(Im(Y)/Re(Y))
>   Yr    <- Re(Y)
>   Yi    <- Im(Y)
>
> #PLOT SIGNALS
>   plot(t,y,type="l",xlim=c(0,T))
>   plot(f,Re(Y),type="l")
>   plot(f,Im(Y),type="l")
> --
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> Sent from the R help mailing list archive at Nabble.com.
>
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>



-- 
Stephen Sefick

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods.  We are mammals, and have not exhausted the
annoying little problems of being mammals.

								-K. Mullis